In digital communication systems such as wired, wireless and optical communication systems, a transmitted signal reaches a receiver after propagation though a channel. In the propagation channel, the transmitted signal interacts with the environment in a very complex way. As an example, signal propagation though a wireless communication channel causes various types of impairment in a received signal due to reflections from large obstacles, diffraction around smaller objects and edges, and refraction through the medium and signal scattering. In a wired channel, such as an ADSL channel, impairments in a received signal may be caused by reflections due to cable mismatching and imperfect terminations. In an optical fibre system, impairments in a received signal are caused due to multiple reflections at connectors and splices. These complex interactions result in reception of multiple delayed versions or replicas of the transmitted signal through multiple paths with each version or replica having randomly distributed amplitude and phase. In general, a channel is known to experience a delay spread (time dispersion) due to multiple paths and is referred to as a multipath fading channel.
Multipath fading may cause errors in signal decoding due to inter-symbol-interference (ISI) and may further affect the performance of communication systems. Therefore, the delay spread is considered to be one of the most important characteristics of propagation channels since it affects the performance of the communication systems. The knowledge about the delay spread of a channel can thus be used for designing a better receiver structure which can adapt itself to deal with the changing nature of the underlying channel and provide an improved performance that enhances the user experience. A large number of delay spread estimation techniques are found in the prior art.
In the U.S. Pat. No. 7,027,527, a method for detecting the presence of an excess delay spread within a received signal is described. In this prior art, a quantitative estimation of a similarity of two segments of the received signal is calculated. The two segments are chosen from synchronization data, where the first segment corresponds to data on even-numbered OFDM (orthogonal frequency domain multiplexing) subcarriers and the second segment corresponds to data on odd-numbered OFDM subcarriers. In determining the presence of the excess delay spread, the quantitative estimation is compared to a detection threshold. A drawback with the delay spread estimation technique disclosed in this prior art is that the method only detects the excess delay spread in the received signal and does not quantify the amount of delay spread in the channel.
In the U.S. Pat. No. 6,028,901, a method of selecting and deselecting an equalizer is performed based on an estimated delay spread in the channel. In this prior art, the path ratio, which is the ratio between the dominant path's power amplitude to the secondary path's power amplitude indicates the amount of delay spread in the received signal. A matched filtering operating is further used to estimate the channel and extract the impulse response of the channel. In addition, a threshold delay spread is determined and the estimated delay spread is compared to the threshold in order to decide whether to select the equalizer or not. A drawback with the method described in this prior art is that the delay spread estimation process is computationally intensive leading to high power consumption of the receiver.